1. Field of the Invention
The present invention relates to new proline derivatives useful as medicaments or intermediates therefor.
2. Description of the Prior Art
Tsuru, Yoshimoto et al. have recently revealed that those substances having a proline skeleton at their N-terminal, e.g. N-benzyloxy-carbonyl-L-prolyl-L-prolinal, have inhibitory activity to proline endopeptidase as well as an anti-amnestic action (Japanese Laid-open Patent Appln No. Sho. 60-188317 and U.S. Pat. No. 4,687,778). These substances are expected by many to open a way to the treatment of senile dementia, for example Alzheimer's dementia which is now increasing, and attempts of synthesis have been made to seek more potent proline endopeptidase inhibitors. (Japanese Patent Appln. Sho. 62-225377 and 62-327181.) Among these groups of compounds, those derivatives, with an appropriate protective group at the N-terminal, whose C-terminal takes the form of an aldehyde group, i.e. di- and mono-peptide derivatives of prolinal and thioprolinal, have been found to possess an outstanding level of such activity.
These derivatives, however, are chemically unstable, for example they are subject to oxidation due to the aldehyde group. There is therefore a demand to develop compounds of higher chemical stability for the purpose of developing medicaments for clinical use.